Impedance matching circuits based on MEMS structures such as switches or relays offer the best performance of reduced insertion loss when varying impedance over the Smith chart. However, a MEMS switch used in an impedance matching circuit such as an antenna tuner may be damaged by a potentially harmful differential voltage across the MEMS switch during a process known as hot switching. Hot switching as applied to a MEMS switch means changing the state of the MEMS switch from open to closed or vice versa while a radio frequency (RF) signal or other signal with a damaging potential is present at a terminal of the MEMS switch. Nevertheless, hot switching is a desirable capability for an impedance matching circuit such as an antenna tuner because tuning adjustments may be performed without interrupting a radio signal being processed.
In particular, a MEMS switch is often harmed when a damagingly large differential voltage creates current surges during a state change of the MEMS switch. For example, as an Ohmic type MEMS switch closes, an electric field due to the RF signal may increase to a point that a damaging electrostatic discharge (ESD) may occur. As a beam of the MEMS switch/relay deflects and comes partially into contact with a signal path section, the RF signal can cause a damaging current surge along with arching. Such a surge in current can damage the beam of the MEMS switch/relay and potentially cause switch failure. Even a very small ESD event can degrade the switch contacts of a MEMS switch. Most MEMS manufacturers specify no more than −10 dBm RF power be present on the terminal of a MEMS switch during a hot switching event. Otherwise, most MEMS manufactures warn that MEMS switch reliability can be adversely affected.
An antenna tuner uses one or more MEMS switches to change the impedance of the antenna tuner. For example, in the switchable pi-network of impedance matching circuit 10, the capacitance is changed. In applications where continuous reception or transmission is required an uncontrollable RF RX blocker signal can be present at the antenna 14 at a relatively high level. In the case of concurrent emission of WLAN or WIMAX the level can be as high as high as +10 dBm. In other instances, the presence of −0 dBm blocker levels may come from various sources of interference such as a broadcast Television station or from purposeful blocking signals in military applications, etc. The level of such signals will create a hot switching condition on the MEMS switches 18A-18C and 30A-30C.
FIG. 1 illustrates an impedance matching circuit 10 based on a pi-network topology that may be damaged during a hot switching event due to an uncontrollable receive (RX) blocker 12 that is coupled to an antenna 14. In particular, the impedance matching circuit 10 is an impedance matching network for matching the impedance of a load, and in this case the antenna 14, to a radio frequency (RF) source 16. In the particular example of FIG. 1, the impedance matching circuit 10 is also known as an antenna tuner.
The impedance matching circuit 10 comprises a first plurality of switch-capacitor branches made from MEMS switches 18A, 18B, and 18C and capacitors 20A, 20B and 20C. A first terminal of each of MEMS switches 18A, 18B, and 18C is coupled to a first terminal of a corresponding one of capacitors 20A, 20B, and 20C. In this way, each switch-capacitor branch has a branch node between each one of MEMS switches 18A, 18B, and 18C and the corresponding one of capacitors 20A, 20B and 20C. Moreover, the first plurality of MEMS switches 18A, 18B, and 18C each have a second terminal that is coupled to a first signal node 22. The capacitors 20A, 20B, and 20C are each selectively coupled through the corresponding one of the MEMS switches 18A, 18B, and 18C to the first signal node 22. The capacitors 20A, 20B, and 20C each have a second terminal coupled to a common node, such as a ground node 24. In this configuration, the first plurality of switch-capacitor branches are in parallel with one another.
The RF source 16 has a first terminal coupled to the ground node 24 and a second terminal coupled to the first signal node 22. An inductor 26 has a first terminal coupled to first signal node 22 and a second terminal coupled to a second signal node 28, which in turn is coupled to the uncontrollable RX blocker 12.
The impedance matching circuit 10 also comprises a second plurality of switch-capacitor branches made from MEMS switches 30A, 30B and 30C and capacitors 32A, 32B and 32C. A first terminal of each of MEMS switches 30A, 30B and 30C is coupled to a first terminal of a corresponding one of capacitors 32A, 32B and 32C. In this way, each switch-capacitor branch has a branch node between each one of MEMS switches 30A, 30B and 30C and the corresponding one of capacitors 32A, 32B and 32C. Moreover, the first plurality of MEMS switches 30A, 30B and 30C each have a second terminal that is coupled to the second signal node 28. The capacitors 32A, 32B and 32C are each selectively coupled through the corresponding one of the MEMS switches 30A, 30B and 30C to the second signal node 28. The capacitors 32A, 32B and 32C each have a second terminal coupled to a common node, such as the ground node 24. In this configuration, the second plurality of switch-capacitor branches are in parallel with one another. Each of the MEMS switches 18A, 18B, 18C, 30A, 30B and 30C may be actuated by an electrostatic charge, thermal, piezoelectric or other actuation mechanism initiated by a control signal.
During hot switching, an RF signal from RX blocker 12 leaks onto the second signal node 28 and the first signal node 22 of impedance matching circuit 10. As a result, there is a potentially damaging difference voltage across each of the MEMS switches 18A, 18B, 18C, 30A, 30B and 30C. While, the risk of permanent failure during an individual hot switching event is relatively small, the odds of a permanent failure for at least one of the MEMS switches 18A, 18B, 18C, 30A, 30B and 30C due to repeated actuation and deactuation over millions of cycles is relatively high. Thus, there is a need to provide the benefits of impedance matching circuits based on MEMS structures that minimize or eliminate the damage potential of hot switching.